The present invention relates to a control circuit for a reluctance machine and to a method of operating such a control circuit.
Reluctance machines have two parts, which are movable in relation to each other, commonly called the xe2x80x9cstatorxe2x80x9d and the xe2x80x9crotorxe2x80x9d, respectively. The most common types of reluctance machines have a rotor that is suspended so that it can rotate inside a stator. The machine is provided with coils, which can be provided with electrical current so as to produce a magnetic flow. The rotor in combination with the stator form a magnetic circuit through which the magnetic field produced by the coils flows.
When the mutual position between the rotor and the stator changes then the reluctance in the magnetic circuit changes.
In order to drive a reluctance motor with a number of windings the current is connected to the windings in a way which depends upon the mutual position of the rotor and the stator.
A known way of providing this control of the current includes sensing the position of the rotor with the help of separate position sensors coupled to the rotor, whereby the position sensors produce an output signal which depends on the position of the rotor.
Another known way of achieving control of phase currents uses the fact that the inductance of a phase varies depending on the position of the rotor in relation to the stator.
The U.S. Pat. No. 5,043,643 describes a method of determining the position of the rotor for a reluctance machine starting from the equation
(Jxe2x88x92Ri)=d/dt(Li),
where i is the current through the phase windings, where U is the voltage across a series connection of the phase winding, a transistor valve and a current sensor resistor, and where R is a predetermined constant corresponding to the sum of the resistances in a phase winding, an activated transistor and a current measuring resistor.
The momentary inductance of a winding can be determined by providing a voltage pulse having a certain amplitude to a winding and measuring the current-time response. Hence, an inductance value can, for example, be calculated by measuring the amplitude of the current at a certain time after switching on the pulse. Alternatively an inductance value can be calculated by measuring the amount of time until a certain current amplitude has been reached. In both cases, however, the measurement involves the provision of voltage pulses to the winding. The provision of such voltage pulses with a certain repetition frequency may cause noise to be generated by vibrations in the components.
The present invention relates to the problem of improving the performance of a control circuit for a reluctance machine.
This problem is solved by a control circuit for a reluctance machine having two mutually movable parts and at least one inductive phase winding, the inductance of which depends on the mutual position of the parts; the control circuit comprising:
a first terminal and a second terminal for connection to a power source;
valve means for setting the winding in a drive mode so as to cause a drive current to flow through the winding, the drive current having a certain maximum value; and
means for establishing said mutual position in dependence of a measured current value and/or time value. According to an embodiment of the invention, the control circuit includes a current sensor for measurement of a test current when the winding is in a non-drive mode, the current sensor being adapted for measurement of test currents having a second maximum value lower than said certain maximum value; wherein
said position establishing means is adapted to establish said mutual position in dependence of said test current.
This solution advantageously enables position measurements using lower current amplitudes. The lower current amplitudes advantageously leads to reduced noise from the reluctance machine, since the noise level is closely connected with the current amplitudes.
Additionally the above solution enables more frequent current measurements, thereby improving the versatility of the control circuit. Since the position and the inductance value, using the above described control circuit, can be established with a lower current top value, the repetition frequency can also be increased. This is due to the fact that a lower top current value through the winding at the end of a voltage pulse corresponds to a lower amount of stored energy in the winding. Hence, the stored energy in the winding can be drained faster and a new measurement cycle can be initiated that much earlier. Thus the inductance measurement procedure can be used at a higher machine speed. According to referred embodiments the current measurement can have a repetition frequency of more than one kilo-hertz. Preferably the current measurement repetition frequency is higher than 1,5 kHz.
With a more frequent current measurement the inaccuracy of the resulting inductance determinations is also reduced, leading to more accurate position determination for a reluctance machine in motion. This positive effect is obtained since the shortened duration of the current measurement procedure, when the reluctance machine is in motion, leads to smaller movement of the rotor during the measurement procedure. Since the inductance is position dependent, the reduced rotor movement leads to reduced inductance change due to movement during the current measurement procedure.
Since the present current measurement solution makes it possible to perform the inductance-measurement using separate current sensors S3A, S3B, S3C it is possible to use lower current values during the measurement for establishing the momentary inductance value of the winding. In other words the ratio between the maximum drive current and the maximum test current is increased. An increase of this ratio is advantageous since the test current may produce a negative torque when the test current flows through a winding which is in a non-drive mode. Hence, a high ratio between the maximum drive current and the maximum test current reduces the impact of any negative torque produced by the test current In effect, an increased ratio between the maximum drive current and the maximum test current renders improved performance of the reluctance machine.
According to an embodiment the control circuit operates to control the valves to prevent currents exceeding said second maximum value from flowing through the current sensor. The second maximum value may be 5 percent of the drive current maximum value.
The control circuit further comprises means for establishing a rate of change for the test current in response to said time value and said current value; said rate of change being indicative of the momentary inductance of the winding. The position establishing means is adapted to establish said mutual position in dependence of said rate of change.